ENABLING AUTOMATION IN MOTION & CONTROL nskautomation.com

During the 21st century, our society has been at the

forefront of discovering new solutions through advanced

technology for medical device and clinical laboratory

applications. These innovative solutions have utilized

linear motion to develop stateof-the-art medical imaging,

diagnostic and surgical equipment. Linear motion is a

common need for all types of advanced equipment and

machines; but precise, smooth, reliable, and repeatable

linear motion is fundamentally important for several

applications within the medical industry. Precision ground

ball screws have become the preferred choice for precise

linear motion because they deliver smooth and accurate

movement ensuring reliable and repeatable results.

NSK TECHNICAL INSIGHT

OVERVIEW

The demand to reduce the size of linear screw actuators, but

maintain precision, is evident in several medical diagnostic

and imaging applications. A diagnostic device, such as a

desktop blood analyzer, can process multiple samples at

a faster speed and higher accuracy in comparison to its

predecessor that was the size of an automobile. Health care

practitioners rely on small ball screw actuators on syringe

pumps to accurately dispense precise levels of medication.

In imaging applications, such as CT scanners, small actuators

position aperture plates to control the X-ray beam. In each

of these applications, a lead screw or ball screw is combined

with a small electric motor to translate the motor’s rotary

motion into linear motion.MBSA used in a Blood Analyzer

MECHATRONIC INTEGRATION FOR PRECISION MEDICAL DEVICE LINEAR MOTIONOPTIMIZING THE BALL SCREW AND MOTOR INTERFACE FOR COMPACT FORM, HIGH EFFICIENCY, AND POSITIONING ACCURACY

MBSA – Motorized Ball Screw Actuator

ENABLING AUTOMATION IN MOTION & CONTROL nskautomation.com

NSK TECHNICAL INSIGHT

LEAD SCREWS VS. PRECISION GROUND BALL SCREWS

In order to achieve linear motion for medical applications,

the traditional solution combines a lead screw with an

electric stepping motor. Lead screws use a V-shaped helical

thread on the shaft with a matching thread inside the nut,

similar to a nut and bolt you can get at a hardware store.

Lead screws (and ACME screws) rely on sliding contact

between the surfaces of the nut thread and the shaft

thread to produce linear motion.

The benefit of a lead screw is the small nut relative to the

axial load it can support due to the large flat contact surface

areas of the mating threads. However, the flat surfaces

sliding across each other generate heat caused by friction.

A large motor is required to overcome friction. As a result,

this solution does not provide smooth consistent motion

and will require more maintenance due to considerable

wear. “Backlash” is inherent with the typical lead screw

design caused by clearance between the nut thread and

the shaft thread. The backlash affects the linear positioning

accuracy of the screw. NSK understands the importance

of minimizing or eliminating backlash and now offers an

alternative solution.

Regarded as the world’s leading manufacturer of precision

ground ball screws, NSK sought to integrate its precision

ground ball screw into an electric stepping motor resulting

in the new MBSA Series. Similar to a lead screw, a ball

screw also has a helical thread on the shaft and inside the

nut; but a ball screw uses precision ball bearings between

the nut and the shaft. Therefore the ball screw thread

profile is different than that of a lead screw. The ball

screw has a U-shaped groove, or Gothic arch, to house the

precision ball bearings. The nut thread profile acts as the

outer raceway and the groove along ball screw shaft acts as

the inner raceway for ball bearing travel.

directionof load

backlash - thread clearancethread contact

Nut

Screw

Lead Screw Nut and Shaft showing Axial and Radial Clearance

Precision Ground Ball Screw Gothic Arch Raceway

Ball Nut Gothic Arch

ScrewShaft

Balls

2

DESIGN FEATURES

›

support bearings

›

›

›

›

Efficiency of Normal Operation: Converting Rotary Motion to Linear Motion

µ: friction coefficient

10

20

30

40

50

60

70

80

90

100

Effi

cien

cy h

1 (%

)

0 1 2 3 4 5 6 7 8 9 10

Lead angle (degree)

µ=0.003

Ball Screw Acme Screw

µ=0.005µ=0.001

µ=0.1

µ=0.2

Ball Screw vs Lead Screw Efficiency

ENABLING AUTOMATION IN MOTION & CONTROL nskautomation.com

NSK TECHNICAL INSIGHT

The ball bearings provide a rolling contact point between

the nut and the shaft that lowers the coefficient of friction.

The result is a highly efficient (90% to 95%) mechanism

that requires less torque to convert rotational motion

into linear motion making ball screws a better fit for the

challenges inherent to medical applications. By using a

precision ground ball screw, the application results in lower

operating temperatures, smoother motion, reduced motor

size, less energy consumed, less wear, and longer life in

comparison to a lead screw solution.

In order to adhere to small size constraints for the nut

body, NSK uses a bridge-type deflector recirculation piece

to guide the ball bearings between adjacent ball thread

grooves or raceways providing the recirculation necessary

for a ball screw. Since the deflectors sit below the surface

of the nut body outside diameter, this area can be used as

a pilot surface for instruments or components attached to

the nut.

How To Prevent Backlash

To eliminate backlash or axial play between the screw shaft

and the nut, NSK uses slightly oversized balls to create a

light preload. The elastic deformation of the balls creates

an internal force between the nut and the screw shaft to

eliminate clearance. The result is precise linear movement

of the object attached to the ball screw nut with zero

backlash between shaft rotation and nut linear movement.

This preload allows for better positioning control for medical

applications with multi-directional axial loads.

High positioning accuracy is the result of a precision ground

ball screw with a preloaded nut that eliminates backlash.

Lead accuracy is measured by calculating the difference

between the actual linear distance traveled by the ball screw

nut in one shaft revolution versus the theoretical distance

traveled (equal to the lead). The International Organization

for Standardization (ISO) has set lead accuracy standards for

precision ball screws. To conceptually visualize the accuracy,

the average human hair is 0.080mm in diameter. For the

MBSA Series, NSK uses a C3 accuracy ball screw which

has a lead accuracy of 0.006mm per shaft revolution. The

stepping motor paired with the ball screw has 200 counts

per revolution resulting in a motor resolution of 0.005mm

for a 1mm lead ball screw. When measuring lead accuracy

over 300mm (or 1 foot) of travel, the actual nut position

deviation from the theoretical nut position is a maximum of

0.008mm for a C3 accuracy ball screw. Comparatively, a lead

screw can have a deviation as large as 0.250mm in 300mm

of travel. Therefore the MBSA Series is the optimal solution

for highly precise medical devices and laboratory equipment.

Ball Nut

oversized ballScrew Shaft

Ball Screw Preload using Oversized Ball Bearing

Ball Screw 4 Points of Contact

deflectorBall Nut

deflector

deflector- bridgetype

Screw Shaft

Deflector-Type Ball Screw

lead

Screw Shaft

Ball Nutlead

4-point contact

ENABLING AUTOMATION IN MOTION & CONTROL Motorized Ball Screw Actuator / ATI / 20

NSK TECHNICAL INSIGHT

Integrating Ball Screws And Motors

In the development of the MBSA Series, NSK replaced the

electric motor output shaft with a journal that is part of the

ball screw shaft. The motor directly rotates the ball screw

shaft which translates the nut for linear motion. This design

eliminates the need for a separate motor-to-ball screw

coupling. The MBSA Series allows for a compact space-

saving design that reduces system inertia and eliminates

alignment error that can occur when the motor and ball

screw are separately mounted. This new product series is

small enough to fit in the palm of your hand.

The MBSA Series is also designed to accommodate a

configurable encoder mounted to the back of the stepping

motor. The stepping motor can communicate with a wide

variety of controllers/drivers. Customers may customize by

selecting the controller/driver of their choice.

Sizing And Availability

As our world continues to evolve, medical devices

and laboratory equipment used in diagnostics, syringe

pumps, microfluidics, imaging, cancer treatment, and

other applications requiring automation will benefit from

these types of integrated solutions. To learn more about

how NSK’s MBSA Series can ensure precise, reliable and

repeatable movement for your medical application,

contact us (1.888.446.5675) or visit our website

(nskautomation.com). NEMA14 MBSA with Optional Encoder and Support Bearing Installed

STANDARD MBSA SCREW SHAFT / MOTOR MODEL COMBINATIONS

SHAFT DIAMETER 06 08

STEPPER MOTOR NEMA 14 NEMA 14 NEMA 17

SHAFT LEAD 1 2 1 2 1 2

THR

EAD

ED S

HA

FT L

ENG

TH 50

100

125

150

200

Shaft Diameter / Stepper Motor / Shaft Lead combinations available from stock. For additional combinations, please consult NSK.